1. Field of the Invention
The present invention relates to heating a fusion spliced portion of optical fibers, after the dissimilar optical fibers having the different mode field diameters or core diameters are fusion spliced. More particularly, the present invention relates to a method and apparatus for heating a fusion spliced portion of optical fibers so that the optical losses associated with the splicing (hereinafter referred to as a splice loss) are low, and an optical fiber array manufactured using the same method and apparatus.
2. Description of the Related Art
In recent years, a hybrid optical fiber has been developed in which a high performance optical fiber having a smaller mode field diameter such as an optical fiber for wavelength division multiplexing transmission or an optical fiber for Raman amplification and a normal single mode optical fiber having a relatively large mode field diameter are combined. In splicing the high performance optical fiber and the normal single mode optical fiber which are different in a mode field diameter or core diameter (hereinafter referred to as a core diameter) of optical fiber, it is difficult to achieve a practical low splice loss simply by fusion splicing. Hence, there is provided a well-known method (Thermally Expanded Core, hereinafter referred to as a TEC) in which the fusion spliced portion of optical fibers is heated and tapered to equalize the core diameters of the splicing portion, and make a smooth splicing shape (refer to Japanese Patent No. 2618500).
FIGS. 9a and 9B are views showing one example of a TEC process including heating a fusion spliced portion. FIG. 9A is a view showing the TEC process of heating the fusion spliced portion of optical fibers using a burner after fusion splicing the optical fibers having different core diameters. FIG. 9B is a view showing a state of the fusion spliced portion of optical fibers after the TEC process as shown in FIG. 9A. In the figures, reference numeral 1a, 1b denotes an optical fiber, 2 denotes a glass fiber portion (cladding portion), 3a, 3b denotes a core portion, 4 denotes the fiber coatings, 5 denotes a fusion spliced portion, 6 denotes a burner, and 7 denotes a core expanded region.
The optical fibers 1a and 1b to be fusion spliced together have the same outer diameter of the glass fiber portion (cladding portion) 2, but are different in the core diameter of the core portions 3a and 3b and the specific refractive index difference. End faces of the optical fibers 1a and 1b to be spliced are disposed oppositely, fused using arc discharge, and butt jointed, as shown in FIG. 9A. Simply by making the fusion splicing, the splicing is discontinuous at the fusion spliced portion 5, because of a difference in the core diameter between the core portion 3a of the optical fiber 1a and the core portion 3b of the optical fiber 1b. This discontinuity causes a large splice loss.
To improve this discontinuity, the TEC process is conducted by heating a vicinity of the fusion spliced portion through the use of a micro torch or the burner 6 with a combustion gas. This heating is made at the temperature and for the time where the optical fibers 1a and 1b themselves are not melted, but a dopant agent, which raises the refractive index, added to the core portions 3a and 3b is diffused to the cladding portion. After this heating process, the dopant agent added to the core portions 3a and 3b is diffused to the cladding portion 2, so that the core diameter of the core portions 3a and 3b is expanded. It diffuses more in case of the optical fiber 1a having a smaller core diameter and a higher dopant concentration than the optical fiber 1b having a larger core diameter and a lower dopant concentration.
By performing the TEC process, the core diameter of the core portion 3a for the optical fiber 1a having smaller core diameter is expanded in taper form, thereby reducing a discontinuity from the core portion 3b of the optical fiber 1b having larger core diameter, as shown in FIG. 9B. In the case where the dissimilar optical fibers are fusion spliced together, it has been found that the TEC process allows the core diameter of the optical fiber having smaller core diameter to gradually approximate the core diameter of the other optical fiber, thereby reducing a splice loss. Also, it has been known that the TEC process by heating is effective to reduce the splice loss due to the core eccentricity by expanding the core diameter of the fusion spliced portion even if similar optical fibers are spliced (refer to Japanese Patent Unexamined Publication No. Sho. 61-117508).